1. Field of the Invention
The present invention relates to switchable electrochromic devices that are capable of uniform switching throughout the entire structure. More particularly, the present invention is directed to switchable electrochromic devices, specifically aircraft window transparencies, which can uniformly switch between an activated and an unactivated state.
2. Technical Considerations
Commercial electrochromic devices are known in the art for use as transparencies, such as automotive windows, automobile mirrors, aircraft window assemblies, sunroofs, skylights, and architectural windows. Such electrochromic devices typically include a sealed chamber defined by two pieces of glass that are separated by a gap or space that contains an electrochromic medium. The electrochromic medium typically includes anodic compounds and cathodic compounds together in a solution. The glass substrates typically include transparent electrically conductive layers coated on facing surfaces of the glass and in contact with the electrochromic medium. The conductive layers on both glass substrates are connected to electronic circuitry. When the conductive layers are electrically energized, an applied potential is introduced into the chamber of the device, which electrically energizes the electrochromic medium and causes the medium to change color. For example, when the electrochromic medium is energized, it can darken and begin to absorb light.
Presently available coated transparencies typically contain one or more metal or metal oxide coatings, such as but not limited to silver, gold, tin oxide, indium tin oxide (ITO), fluorine doped tin oxide (FTO), antimony doped tin oxide, and ITO/metal/ITO (IMI), as the conductive coating on glass substrates. Electrochromic devices preferably have a sheet resistance on the order of 2 ohms per square for more rapid switching. However, 2 ohms per square of presently available conductive coatings, such as ITO, has an undesirable thickness and provides for decreased light transmittance. Excessive thickness of the conductive coating can result in buckling and subsequent separation of the conductive coating from the substrate surface, which can lead to costly repairs, or replacement, of the device. Therefore, it would be advantages to provide electrically conductive metal or metal oxide coatings for electrochromic devices that do not have the above problems.
In a conventional electrochromic window, a primer layer is typically provided on the glass substrate to improve the adhesion between the glass and the subsequently applied conductive coating, e.g. ITO. However, glass can crack and increases the weight of the electrochromic device. It would be desirable, therefore to utilize lighter weight materials, such as plastics, to replace the glass substrates of conventional electrochromic window to provide electrochromic devices that are lightweight, relatively inexpensive and durable. However, one problem associated with plastic substrates is that plastic has a larger coefficient of expansion than glass. Thus, primers used with glass substrates have a tendency to crack and break when placed on plastic substrates due to compressive and expansive stresses.
As can now be appreciated, it would be desirable to provide a primer coating that could be utilized on a plastic substrate of an electrochromic device to improve the adhesion between the plastic and the subsequently applied conductive coating, e.g. ITO.